The present invention relates to a process for producing a titanium oxide, specifically a process for producing a titanium oxide exhibiting a photocatalytic activity.
Light irradiation to a photocatalyst generates electrons having a strong reduction activity and positive holes having a strong oxidation activity, to decompose a molecular species that comes in contact with the photocatalyst by an oxidation-reduction activity. Such an activity is called a photocatalytic activity. By the photocatalytic activity, NOx in the atmosphere is decomposed, bad-smelling substances, molds or the like in a living or working space are decomposed and removed, and environmental pollution substances such as organic solvents, agrochemicals and surfactants in water are decomposed and removed. In these days, a photocatalyst exhibiting the photocatalytic activity by irradiation of visible light has been researched from the point of general purpose and usage. It has been known that a certain titanium oxide can be utilized for such a photocatalyst.
Some processes for producing a titanium oxide exhibiting the photocatalytic activity have been proposed. For example, PCT International Application Laid-Open No. WO98/23374 discloses that a titanium oxide is treated in a microwave low-temperature plasma method so that a titania with a carbon deposit is formed thereon. In this method, however, there are problems in that a specific apparatus having a vacuum vessel such as a microwave low-temperature plasma generator is needed and the process thereof is complicated.
The objects of the present invention is to provide a process for easily producing a titanium oxide that shows sufficiently high photocatalytic activities by irradiation of visible light without utilizing a specific apparatus having a vacuum vessel.
The present inventors have studied on such a titanium oxide As a result, the present inventors have found that the titanium oxide can be easily produced by a process comprising a step of calcining a titanium compound under a specific condition, and have completed the present invention.
Thus, the present invention provides a process for producing a titanium oxide which comprises (i) a step of calcining a titanium compound in the presence of ammonia gas or (ii) steps of treating a titanium compound with heat in the presence of ammonia gas and calcining the heat-treated titanium compound.
A titanium oxide in the present invention is produced by a process which comprises (i) a step of calcining a titanium compound in the presence of ammonia gas or (ii) steps of treating a titanium compound with heat in the presence of ammonia gas and calcining the heat-treated titanium compound.
Examples of the titanium compound used in the present invention include titanium hydroxides, titanic acids, other inorganic titanium compounds, organic titanium compounds, partially hydrolyzed inorganic titanium compounds and partially hydrolyzed organic titanium compounds. The partially hydrolyzed titanium compounds may be obtained by hydrolyzing the corresponding titanium compounds with smaller amount of water (which may be in a liquid state or vapor state) than that in a stoichiometric amount.
Examples of the titanium hydroxides and titanic acids include titanium hydroxide (II) (Ti(OH)2), titanium hydroxide (III) (Ti(OH)3), titanium hydroxide (IV) (Ti(OH)4), titanium oxyhydroxide (TiO(OH)2), xcex1-titanic acid (ortho-titanic acid (H4TiO4)), xcex2-titanic acid (meta-titanic acid (H2TiO3)) and the like. Examples of other inorganic titanium compounds include titanium trichloride, titanium tetrachloride, titanium tetrabromide, titanium sulfate, titanium oxysulfate and the like. Examples of organic titanium compounds include tetra-alkoxy titanium compounds such as tetra-isopropoxy titanate, tetra-n-butoxy titanate, tetrakis(2-ethylhexyloxy) titanate and tetrasteariloxy titanate; titanium acylate compound; titanium chelate compounds such as di-isopropoxy-bis(acetylacetonato) titanium complex, isopropoxy(2-ethyl-1,3-hexandiorato) titanium complex, hydroxy-bis(lactato) titanium complex and the like.
In the present invention, the titanium hydroxides and the partially hydrolyzed organic titanium compounds are preferably used as the titanium compound. It is preferred that the titanium compound contains that having an amorphous phase. The more a rate containing amorphous titanium compound increases, the higher photocatalytic activity the resulting titanium oxide tends to exhibit when radiated with visible light. The ratio of the amorphous titanium compound to the whole titanium compound is measured in an X-ray diffraction method. The titanium hydroxide containing that of an amorphous phase may be obtained by hydrolyzing the above-described other inorganic titanium compound, the organic titanium compound or the like.
When a titanium hydroxide is utilized as the titanium compound in the present invention, it is preferred that the titanium hydroxide contains nitrogen or a nitrogen compound. When the titanium hydroxide containing nitrogen or the nitrogen compound in a larger amount is utilized, the resulting titanium oxide tends to exhibit a higher photocatalytic activity. The amount of nitrogen, the nitrogen compound or the like, which is contained in the titanium hydroxide, may be preferably about 0.2% by weight, more preferably about 1% by weight, much more preferably about 2.5% by weight in terms of nitrogen atom based on the titanium hydroxide. The amount of nitrogen, the nitrogen compound or the like may be measured by a melting method using a nitrogen analyzer.
The titanium hydroxide containing nitrogen or the nitrogen compound may be obtained in a method of adding an alkaline compound such as ammonia solution (ammonia water) or an alkali hydroxide into a titanium oxysulfate solution to precipitate a titanium hydroxide, or in a method of adding water or ammonia solution into a titanium chloride, to hydrolyze the titanium compound.
The titanium compound used in the present invention may be obtained after a mold processing. Examples of the shape of such a molded titanium compound include powdery shape, fibrous shape, thin-layer shape and the like. When the molded titanium compound having a designated shape formed by the mold processing is utilized in the present invention, the resulting titanium oxide exhibits sufficiently high photocatalytic activity as well as it has a designated shape. For example, when a fibrous-shaped titanium compound is used, a fibrous-shaped titanium oxide is provided. When a thin-layer-shaped titanium compound is used, a thin-layer-shaped titanium oxide is provided.
Alternatively, the above-described mold processing may be conducted in a method of dispersing a titanium compound in a solvent such as water or an alcohol to obtain a slurry, applying the slurry onto an uneven surface of a metal plate and drying the slurry. By such a method, a titanium hydroxide having a specific shape or a thin-layered shaped titanium hydroxide having a specific surface can be obtained.
In the case that an ammonia gas is present in calcinating a titanium compound, the ammonia gas may be allowed to be present by a method in which the titanium compound is treated with heat in the present of an ammonia gas and then calcined in a nitrogen atmosphere or in air, or by a method in which a compound which generates ammonia in calcination is allowed to come in contact with a titanium hydroxide before and/or in calcining the titanium hydroxide, or by a method in which an ammonia gas is introduced into a calcination furnace while calcining the titanium compound.
When a titanium compound is treated with heat in the presence of ammonia gas before calcining the titanium compound, such a heat treatment may be carried out by heating the titanium compound at a temperature of about 50xc2x0 C. or higher, preferably about 100xc2x0 C. or higher, and at a temperature of about 200xc2x0 C. or lower, preferably about 150xc2x0 C. or lower. In the heat treatment, a suitable content of ammonia in the ammonia-containing gas may vary depending on a temperature for the heat treatment, a kind of titanium compound to be treated and the like. The content may be about 1% by volume or more, preferably about 10% by volume or more and more preferably about 30% by volume or more.
The heat treatment may be conducted in a method of introducing an ammonia gas into a reaction vessel charged with a titanium compound, followed by heating, or in a method of charging a reaction vessel with a titanium compound and a compound which generates ammonia in calcinations, followed by heating. When a titanium chloride, a titanium bromide, an organic titanium compound or the like is used in the present invention, the heat treatment is preferably conducted in the presence of steam. Stream may be introduced into the reaction vessel during the heat treatment or may be generated during the heat treatment by evaporating an ammonia solution, which has been put in the reaction vessel before the heat treatment. The titanium compound to be treated with heat may have a fibrous shape.
Examples of the compound which generates ammonia in calcination, which is utilized before and/or in the calcinig step, or in the heat treatment, include ammonia solution (NH4OH); an ammonium compound such as ammonium sulfate, ammonium chloride, ammonium carbonate, ammonium nitrate, ammonium phosphate, ammonium acetate; urea; a titanium-containing ammonium compound such as basic titanium ammonium sulfate ((NH4)2 SO4). TiOSO4). Among them, ammonia solution or ammonium sulfate is preferably used.
When a titanium hydroxide comes in contact with ammonia solution before calcination, such a contact of titanium hydroxide with ammonia solution may be carried out in a method of immersing the titanium hydroxide into ammonia solution, or in a method of spraying the titanium hydroxide with ammonia solution. A content of ammonia in the ammonia solution may be about 1% by weight or more, preferably about 5% by weight or more based on the ammonia solution. A temperature in the contact may be about 0xc2x0 C. or higher, preferably about 50xc2x0 C. or higher, and about 200xc2x0 C. or lower, preferably about 100xc2x0 C. or lower. The contact step of the titanium hydroxide with ammonia solution is preferably conducted in a sealed-type pressure reaction vessel in order to prevent the ammonia content from decreasing due to the evaporation of ammonia in ammonia solution.
When a titanium hydroxide comes in contact, before and/or in the calcining step, with the compound which generates ammonia in calcination, an amount of ammonia-generating compound to be used may be about 0.1% by weight or more, preferably about 0.3% by weight or more, more preferably about 1% by weight or more, and may be about 10% by weight or less, preferably about 5% by weight or less in terms of nitrogen atom in the ammonia-generating compound based on the titanium hydroxide substantially containing no water. The titanium hydroxide substantially containing no water may be obtained by drying the titanium hydroxide (with which the ammonia-generating compound comes in contact) at 70xc2x0 C.
A temperature for the calcination of titanium compound maybe about 300xc2x0 C. or higher, preferably about 350xc2x0 C. or higher, and about 600xc2x0 C. or lower, preferably about 500xc2x0 C. or lower. When the calcination temperature is too high, the resulting titanium oxide obtained after the calcination may exhibit a low photocatalytic activity.
When the titanium compound is calcined without the heat treatment in the presence of ammonia gas, the calcination of titanium compound may be conducted in the presence of ammonia gas. In this case, the calcination is conducted in an atmosphere of ammonia-containing gas which has an ammonia content of about 0.1% by volume or more. In the atmosphere of ammonia-containing gas, a content of nitrogen oxide may be about 100 ppm or less, preferably about 50 ppm or less in terms of NOx.
The calcination of titanium compound may be conducted by a method in which a titanium compound is treated with heat in the presence of ammonia gas in a reaction vessel, the ammonia gas is discharged therefrom and then the titanium compound is calcined in the vessel, or by a method in which a titanium compound is treated with heat in the presence of ammonia gas in a reaction vessel, the titanium compound is recovered therefrom after cooling and then the obtained titanium compound is calcined in another vessel, or by a method in which a slurry containing a titanium hydroxide and ammonia solution is prepared, the ammonia solution is removed from the slurry by filtration or centrifugation to obtain titanium hydroxide, followed by drying and then the dried titanium hydroxide is calcined with a furnace such as an electric furnace. Alternatively, the calcination of titanium compound may be conducted by a method in which an ammonia-containing gas is introduced into a calcination furnace which has been charged with a titanium compound and then the temperature of the furnace is raised to a prescribed temperature and the prescribed temperature is maintained to calcine the titanium compound, or by a method in which a calcination furnace which has been charged with a titanium compound is heated to a prescribed temperature while introducing an ammonia-containing gas therein and the prescribed temperature is maintained to calcine the titanium compound. In these methods, the temperature may be raised at a rate of 100xc2x0 C/h or more, preferably 200xc2x0 C./h or more.
The titanium oxide obtained in the present invention may have a crystal structure of anatase crystallinity. The titanium oxide having anatase crystallinity exhibits a superior photocatalytic activity by irradiation of visible light. The titanium oxide, as it is or after a mold processing, can be used for a photocatalyst which is applied for decomposing NOx in the atmosphere; decomposing and removing bad-smelling substances, molds or the like in a living or working space and; decomposing and removing environmental pollution substances such as organic solvents, agrochemicals and surfactants in water.
Also, the titanium oxide obtained in the present invention exhibits a high photocatalytic activity by irradiation of visible light and has various types of shapes and, therefore, the titanium oxide can meet requirements for catalytic materials in a household electric field, a construction field, an automobile field and the like. For example, when an air-purifier filter is made from the fibrous titanium oxide obtained in the present invention, then the filter prevents the titanium oxide from dropping off from the filter to keep its high photocatatytic activity for a long period of time.
As described above, in accordance with the present invention, the titanium oxide exhibiting a high photocatalytic activity by irradiation of visible light is easily produced from a titanium compound.
The process for producing the titanium oxide in the present invention is described in Japanese application nos. 2000-098704 filed on Mar. 31, 2000, 2000-098706 filed on Mar. 31, 2000, 2000-122126 filed on Apr. 24, 2000 and/or 2000-176519 filed on Jun. 13, 2000, the complete disclosures of which are incorporated herein by reference.